Chemical vapor deposition (CVD) is one process for forming thin films on semiconductor wafers, such as films of elemental metals or compounds. CVD involves the formation of a non-volatile solid film on a substrate by the reaction of vapor phase reactants (precursors) that contain desired components of the film. Standard CVD processes use a precursor source in a vaporization chamber of a CVD apparatus. The vaporization chamber is connected to a process (or reactor) chamber wherein a deposition substrate, such as a semiconductor wafer, is located.
CVD (and other thin film vapor deposition) techniques require delivery of a controlled mass of the precursor in the vapor phase. Precise control over the mass of the precursor delivered to the process chamber is needed to form a uniform layer of the desired thin film. In addition, the manner of delivery of the precursor must avoid decomposition of the reactive volatile precursor molecules and must not include unwanted volatized elements or compounds.
Conventional methods of providing a source of vapor-phase precursor molecules include (1) direct vaporization of the precursor from neat solids or liquids, (2) direct vaporization of a solvent containing the precursor, and (3) distilling precursor molecules from a solvent by bubbling a carrier gas through a volume of the solvent containing the precursor.
Bulk sublimation of a solid precursor and transport of the vaporized solid precursor to the process chamber using a carrier gas has been practiced. However, it is difficult to vaporize a solid at a controlled rate such that a constant and reproducible flow of vaporized solid precursor is delivered to the process chamber. Lack of control of the rate of delivery of a vaporized solid precursor is (at least in part) due to a changing surface area of the bulk solid precursor as it is vaporized. The changing surface area of the solid precursor when it is exposed to sublimation temperatures produces a continuously changing rate of vaporization. This is particularly true for thermally sensitive compounds. The changing rate of vaporization thus results in a continuously changing concentration and non-reproducible flow of vaporized precursor delivered for deposition in the process chamber. As a result, film growth rate and the composition of films deposited using such techniques are not adequately controlled. Further, sublimation of solid precursors requires exposure of the precursor to temperatures greater than the vaporization temperature. Many precursor materials decompose when quickly heated to such temperatures.
Liquid precursors may be vaporized directly using a bubbler device. A liquid precursor is heated in a reservoir to a temperature at which there is sufficient vaporization to maintain a particular deposition rate. A stream of carrier gas is directed over the precursor or is bubbled through the liquid precursor in the reservoir. The carrier gas transports vaporized precursor molecules to a process chamber for deposition of a CVD thin film. However, many desirable precursor molecules, when heated to a temperature sufficient to maintain a particular deposition rate will simply decompose in the bubbler.
It is also possible to dissolve a liquid or solid precursor in a solvent and vaporize the solution directly. (Many desirable precursors are solids at room temperature). In the vaporizer (the inlet to which often contains a needle or small orifice), the solvent and the precursor are quickly heated to the gas phase. One of the problems associated with this technique is that the high temperatures necessary to quickly vaporize the solution cause solvent and precursor molecules to decompose. Decomposition of the solvent and precursor molecules within the vaporizer typically produces particulates that clog or otherwise obstruct the delivery lines between the precursor reservoir and the process chamber. Obstruction of the delivery lines cause inconsistent delivery rates of precursor for deposition on the substrate. In addition, the conventional CVD solvents used to dissolve such precursors typically result in CVD processes where the solvent molecules are carried along with the precursor. Additionally, such solvent molecules have a tendency to decompose, further obstructing the delivery lines or may be deposited on the substrate. Solvent decomposition products, e.g., carbonates, formed in the thin film result in poor thin film quality.
As an alternative, liquid or solid precursors may be mixed with or dissolved in a conventional CVD solvent and the solvent containing the precursor placed in a bubbler device. The solvent containing the dissolved precursor is then heated in a reservoir. As described above for liquid precursors, a stream of carrier gas is directed over or bubbled through the solvent. The carrier gas transports the volatile precursor molecules from the solvent to a process chamber. The advantage of this technique is that most precursor elements or compounds may be vaporized in a bubbler device at lower temperatures than required for sublimation or direct vaporization of the precursor. Additionally, control of mass delivery of the precursor, using a bubbler device, is typically better than other precursor vaporization methods. Unfortunately, available CVD solvents are typically organic compounds possessing vapor pressures of greater than about 1 Torr at about room temperature. Accordingly, volatilized solvent molecules are often transported to the process chamber along with the precursor molecules. This problem is exacerbated when temperatures above room temperature are needed to volatilize sufficient precursor molecules and/or to maintain a given depositon rate. As a result, solvent molecules or solvent decomposition products are deposited in the thin-film.
Further, known CVD solvents do not dissolve the range of solid precursors necessary to form the CVD thin films currently in demand. Moreover, many of the known CVD solvents for precursor materials are corrosive to the CVD apparatus, the substrate, and/or thin films already formed on the substrate.
Accordingly, methods and apparatus that take advantage of the benefits of using a bubbler (i.e., lower temperatures and increased control of precursor delivery rates), but overcome the limitations imposed by conventional CVD solvents are needed. CVD methods and apparatus that do not lead to transport of solvent molecules along with the vaporized precursors are needed. That is, CVD methods and apparatus are needed that include solvents having extremely low or substantially no measurable vapor pressure. Additionally, CVD methods and apparatus that may be used along with a conventional bubbler device technology would be preferred. In order to increase the range of precursors that may be used to deposit CVD thin films, CVD methods and apparatus including solvents that exhibit a wide liquid-temperature range and that are resistant to decomposition at relatively high-temperature levels, are needed. Further, CVD methods and apparatus that include solvents that are relatively inert and that dissolve a variety of precursor materials having a wide range of polarities, are needed.